The present invention relates to an image recording apparatus by which an electrostatic latent image is developed with a 2-component developer composed of toner and carrier, and more particularly relates to an electrostatic recording method by which the electrical charge given to the toner is controlled so as to provide an appropriate image density.
In a developing device by which an electrostatic latent image on an image carrier such as a photoreceptor drum is visualized with 2-component developer composed of toner and carrier, the mixing ratio by weight of toner to carrier in the 2-component developer, which will be referred to as toner concentration T.sub.c hereinafter, has great influence on the developing property. For example, in the case where toner concentration of 2-component developer is lower than an appropriate value, the image density obtained by development is lowered. On the contrary, in the case where toner concentration is excessively high, the image density obtained by development is increased too high, so that fogging is caused on the image. As a result of the foregoing, an appropriate recorded image can not be provided on a transfer sheet when the developed image is transferred onto the surface of the transfer sheet.
Accordingly, in order to always provide an image of desirable density, it is necessary to set the toner concentration of 2-component developer to an appropriate level and to maintain it. Conventionally, a toner concentration control system for 2-component developer has been proposed in which the toner concentration of 2-component developer is maintained constant by controlling the amount of supplied toner. The aforementioned toner concentration control system is operated in the following manner: when changes are detected, such as changes in permeability of 2-component developer, volume of the developer, image density after development, and color of the 2-component developer, toner concentration is detected; and according to the detected toner concentration, an amount of toner to be supplied is controlled so that the toner concentration can be maintained on an appropriate level.
However, in the aforementioned toner concentration detecting system in which changes are detected, it is difficult to carry out the detecting operation stably over a long period of time because it is difficult to prevent misdetection and also it is difficult to compensate for the deterioration of the photoreceptor surface that progresses with age.
A method to solve the problem is disclosed in Japanese Patent Publication Open to Public Inspection No. 136669/1982 "Concentration Control Device for 2-component Developer". This concentration control device for 2-component developer is composed as follows:
In order to stably maintain the developing performance of an image recording apparatus over a long period of time, a plurality of detection levels to detect the volume of 2-component developer in the developing unit are provided. In order to detect the developing performance, the density of a developed image or that of an image formed on the photoreceptor surface from a reference image is optically detected. According to the detected image density, the plurality of detecting levels are changed over by a central computing unit so that the toner concentration of 2-component developer can be controlled in order to maintain the developing performance as stably as possible.
The object has been generally accomplished by the aforementioned apparatus.
However, the blackening ratio of a document to be recorded fluctuates due to the difference in image density. Accordingly, in the case where documents of a low blackening ratio such as a line image or a character image are continuously developed so as to form images, a predetermined image density corresponding to the toner concentration can not be provided although the toner concentration is maintained constant, so that the relation between toner concentration and image density can not be maintained. It has been found that the aforementioned phenomenon tends to occur in an electrostatic color recording apparatus by which color images can be reproduced with 2-component developer of various colors.
The inventors have investigated the cause of the aforementioned problem, and obtained the following knowledge.
The inventors have analyzed the aforementioned phenomenon that will become a problem in the image recording operation. As a result of the investigation, they have found that: a developing sleeve and a stirring means are continuously rotated in the developing unit during the image forming process conducted by the electrostatic recording apparatus; and the aforementioned problem relates to the stirring time in the developing unit, that is, the problem relates to the 2-component developer remaining time in which the 2-component developer remains in the toner hopper (hereinafter, called the toner remaining time). In the case of a document of low density, the toner consumption per unit time is small compared with that of a usual case. Accordingly, the toner remaining time is increased. In the case where a difference is caused in the toner consumption of 2-component developer of each color, the toner remaining time is different at each color.
The aforementioned phenomenon occurs even when the toner concentration is constant. Therefore, it can be guessed that the aforementioned phenomenon is not caused by a condition of "spent-toner" in which toner particles are deposited around carrier particles so that the toner charge amount is reduced. It can be considered that: since the aforementioned phenomenon relates to the toner remaining time, toner and carrier particles rub each other over a long period of time when the toner remaining time becomes longer so that the toner charge amount is increased; and when the toner charge amount is increased beyond a predetermined range, it affects image density. Therefore, an investigation has been made to find the relation between the toner charge amount and the image density.
FIG. 20 is a graph showing the change of image density CD with respect to the change of toner charge amount Q/m (.mu.C/g) in the case where the toner concentration in 2-component developer is constant.
The graph in FIG. 20 shows that image density CD is lowered as toner charge amount Q/m is increased. Specifically, when toner charge amount Q/m is 10 (.mu.C/g), image density CD is 1.4. When toner charge amount Q/m is 20 (.mu.c/g), image density CD is 1.0. When toner charge amount Q/m is 30 (.mu.C/g), image density CD is 0.5.
Specific numerals showing the relation between the image density CD and toner charge amount Q/m are different according to the specific conditions such as the developer to be used. In general, the relation can be determined by a force (referred to as developing force, hereinafter) applied to toner in a developing region, the toner being contained in 2-component developer magnetically carried by a developing sleeve in which a permanent magnet is provided.
When developing force is represented by F.sub.t, it can be expressed by the following equation. EQU F.sub.t =q.sub.t .multidot.E-k(q.sub.t .multidot.q.sub.c /r.sup.2)-q.sub.t (V.sub.B /R) (1)
where q.sub.t is a toner charge amount, E is an electrical field caused by an electrostatic latent image (referred to as a latent image electrical field), q.sub.c is a carrier charge amount, r is a distance between toner and carrier, V.sub.B is a voltage impressed upon the developing sleeve (referred to as developing bias), and R is a distance between the surface of the photoreceptor and that of the developing sleeve.
Equation 1 shows that developing force F.sub.t can be obtained when an adhesion force (referred to as Coulomb force) caused between toner and carrier, and a developing bias force applied to toner are subtracted from a force caused by latent image electric field E.
It should be noted here that: a force to determine developing force F.sub.t is proportional to toner charge amount q.sub.t (Q/m); and Coulomb force that decreases developing force F.sub.t tends to increase more than other forces because carrier charge amount q.sub.c is increased as toner charge amount q.sub.t (Q/m) is increased. Consequently, when toner charge amount q.sub.t (Q/m) is increased, developing force F.sub.t is lowered, and when toner charge amount q.sub.t (Q/m) is decreased, developing force F.sub.t is increased .
FIG. 21 is a graph showing the change of image density CD with respect to the toner concentration (wt %) in 2-component developer loaded in the developing unit.
In this case, the 2-component developer includes nonconductive magnetic carrier (coated with resin, and a magnetic particle resin dispersion type), the average particle size of which is 20 to 100 .mu.m, and toner, the average particle size of which is 5 to 15 .mu.m.
In FIG. 21, solid line A shows a case in which developing bias V.sub.B is 200 V. As shown by solid line A, when toner concentration is 1 wt %, image density CD is about 0.4, and when toner concentration is 5 wt %, image density CD is about 1.0, and when toner concentration exceeds 5 wt %, image density CD is saturated at about 1.2. According to this solid line A, image density CD varies from about 0.4 to 1.2 so that the contrast is sufficiently wide, and the adhesion force between toner and the photoreceptor drum is sufficient, and further an amount of image shift is small during the process of image formation . Therefore, it is preferable that image density CD is controlled in accordance with solid line A.
In FIG. 21, one-dotted chain line B represents a relation between toner concentration T.sub.c (wt %) and image density CD in the case where 2-component developer has been stirred over a long period of time so that toner charge amount q.sub.t (Q/m) is increased too much compared with the case shown by solid line A. In this case, other conditions such as developing bias V.sub.B are the same as those of solid line A. As can be seen from expression (1), since Coulomb force is increased compared with other electrostatic forces, the attraction force to attract the toner to the photoreceptor drum is relatively lowered and the image density is decreased. Specifically, it is shown that one-dotted chain line B is saturated at a point where toner concentration is 5 wt % and the image density is 0.5. Consequently, sufficient image density CD corresponding to the document density on line B can not provided, and sufficient contrast can not be provided, either.
Dotted line C represents a relation between toner concentration T.sub.c (wt %) and image density CD in the case where the toner charge amount is reduced compared with solid line A due to environmental fluctuation such as an increase in relative humidity. In this case, it is shown that the adhesion force (Coulomb force) between toner and carrier is not sufficient, so that image density CD is increased compared with a case shown by solid line A. Specifically, image density CD reaches 0.7 when the toner concentration is about 1 wt %, and as the toner concentration (wt %) is increased, it is saturated at about 1.4. Therefore, in the case of development carried out on dotted line C, sufficient contrast width can be provided, however, a large amount of toner is scattered, so that too much toner is deposited on the latent image, which can not be put into practical use from the viewpoint of faithful reproduction of a document image. Further, since the adhesion force between the photoreceptor drum and toner is not sufficiently high, image shift tends to occur.
Further, problems caused when a color image is recorded are stated as follows.
FIG. 34(a) is a graph showing the amount of consumed toner with respect to the number of copies in a color image forming apparatus by which color images are recorded with 2-component developer of a plurality of colors. FIG. 34(b) is a graph showing the toner charge amount of each color corresponding to the number of copies in a color image forming apparatus. FIG. 34(c) is a graph showing optical image density (CD) corresponding to the number of copies in a color image forming apparatus.
In this case, optical image density CD is defined as image density corresponding to reflected light sent from a reference toner image formed in the following manner: an optical scanning system irradiates the surface of a photoreceptor drum with exposure light in accordance with an image signal corresponding to reflected light sent from a reference density plate; and the exposed portion is developed under a predetermined electrostatic process condition to obtain the reference toner image.
FIG. 34(a) shows an example in which a color document is copied. Solid line BK shows an amount of consumed toner corresponding to the number of copies of black toner. Specifically, the following amount of toner is consumed: in the case of A4-size paper, the average toner consumption is 80 mg/sheet, so that 240 g of toner is consumed to copy 3000 sheets. Solid line Y shows an amount of yellow toner consumption corresponding to the number of copies. Specifically, the average toner consumption is 50 mg/sheet, so that 150 g of toner is consumed to copy 3000 sheets. Solid line C shows an amount of cyan toner consumption corresponding to the number of copies. Specifically, the average toner consumption is 50 mg/sheet, so that 150 g of toner is consumed to copy 3000 sheets. Solid line M shows an amount of magenta toner consumption corresponding to the number of copies. Specifically, the average toner consumption is 10 mg/sheet, so that 30 g of toner is consumed to copy 3000 sheets. The graph in FIG. 34(a) shows that the amounts of consumed toner are different according to the color of developer.
An ideal case is explained above in which the developing property of each color is not changed even after 3000 copies have been made.
However, in a common image recording apparatus, the developing sleeve and the stirring means provided in the developing unit are continuously rotated during developing in the image forming process in order to simplify the control of the apparatus. Accordingly, toner remains in the developing unit over a long period of time, so that the toner charge amount of each color is varied. An example in which the toner charge amount is changed will be explained as follows.
In FIG. 34(b), solid line BK shows a change of black toner charge amount Q/m with respect to the number of copies. As can be seen from the graph, the toner charge amount is approximately constant irrespective of the increase in the number of copies. The reason why the toner charge amount is approximately constant, is as follows: since toner concentration is controlled, a necessary amount of toner is successively supplied in accordance with the amount of consumed toner, so that a small amount of toner remains in the developing unit over a long period of time.
Solid line Y shows a change of yellow toner charge amount Q/m with respect to the number of copies. Solid line C shows a change of cyan toner charge amount Q/m with respect to the number of copies. According to solid lines Y and C, it can be seen that the toner charge amount is slightly increased as the increase in the number of copies. The reason why the toner charge amount is slightly increased, is presumably as follows: since toner concentration is controlled, a necessary amount of toner is successively supplied in accordance with the amount of consumed toner, however, a very small amount of toner remains in the developing unit over a long period of time exceeding an allowable range.
Solid line M shows a change of magenta toner charge amount Q/m with respect to the number of copies. Compared to solid line M, lit can be seen that the magenta toner charge amount is extremely increased in accordance with the increase in the number of copies. The reason why the toner charge amount is extremely increased is presumably as follows: since toner concentration is controlled, a necessary amount of toner is successively supplied in accordance with the amount of consumed toner, however, a large amount of toner remains in the developing unit over a long period of time exceeding an allowable range.
When new developer of each color is provided into the developing unit, a difference of the toner charge amount between developers is small so that the difference does not affect the developing properties. However, as the number of copies is increased, a difference is caused between the amounts of consumed toner. Therefore, amounts of toner remaining in the developing unit over a long period of time are different, so that a difference is caused between the toner charge amounts, and as a result, a difference is caused between the developing properties of toners.
Since the difference is caused between the charge amounts of toners, the following phenomena occur. In FIG. 34(c), solid line BK represents a change of optical image density (CD) of black toner corresponding to the number of copies. As shown in the graph, optical image density (CD) of black approximately constant irrespective of the increase in the number of copies. That is, the developing property is approximately constant. Solid line Y shows a change of optical image density (CD) with respect to the number of copies. Solid line C shows a change of optical toner density (CD) with respect to the number of copies. According to solid lines Y and C, it can be seen that optical image density (CD) is slightly increased with the increase in the number of copies, which shows that the developing properties of yellow and cyan toners are a little lowered. Solid line M shows a change of optical image density with respect to the number of copies. Compared to solid line M, it can be seen that optical image density (CD) is extremely increased in accordance with the increase in the number of copies, which shows that the developing properties of magenta toner is extremely lowered.
As described above, in case of an image forming apparatus in which the developer is continuously stirred in the developing unit, amounts of consumed developers are different, so that the remaining time of each developer is different. As a result, the stirring time of each developer is different. Therefore, the developing properties are affected. Consequently, the following problems are caused: When an image of a single color is reproduced, the image density is lowered. In the case of a color copying operation, a predetermined amount of each toner is not provided for development, so that the colors of an image become ill-balanced.